undocumented feature ⇝

feature

1. A good property or behaviour (as of a program).
Whether it was intended or not is immaterial.

2. An intended property or behaviour (as of a program).
Whether it is good or not is immaterial (but if bad, it is
also a misfeature).

3. A surprising property or behaviour; in particular, one that
is purposely inconsistent because it works better that way -
such an inconsistency is therefore a feature and not a
bug. This kind of feature is sometimes called a miswart.

4. A property or behaviour that is gratuitous or unnecessary,
though perhaps also impressive or cute. For example, one
feature of Common LISP's "format" function is the ability to
print numbers in two different Roman-numeral formats (see
bells, whistles, and gongs).

5. A property or behaviour that was put in to help someone
else but that happens to be in your way.

6. A bug that has been documented. To call something a
feature sometimes means the author of the program did not
consider the particular case, and that the program responded
in a way that was unexpected but not strictly incorrect. A
standard joke is that a bug can be turned into a feature
simply by documenting it (then theoretically no one can
complain about it because it's in the manual), or even by
simply declaring it to be good. "That's not a bug, that's a
feature!" is a common catch-phrase. Apparently there is a
Volkswagen Beetle in San Francisco whose license plate reads
"FEATURE".